Mice turn into killers when brain circuit is triggered by laser

Inside every mouse lurks a natural-born killer. Researchers have identified the brain region that controls hunting, and have found a way to switch it on and off.

Ivan de Araujo at Yale University and his team have discovered that two sets of neurons in the mouse brain underpin the instinct to kill. One coordinates the pursuit of prey, while the other controls the neck and jaw muscles used for biting. Both are in the amygdala – the part of the brain involved in motivation, emotion and fear.

By modifying these neurons so they could be activated by laser light, a technique called optogenetics, the team were able to switch these circuits on whenever they wanted.

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When the laser was off, the mice padded around their cages normally. But turning the laser on caused the mice to suddenly launch frenzied attacks on almost anything in their path: live crickets, fake insects or even sticks and bottle caps. After jumping on the “prey”, the mice grabbed it with their paws and repeatedly sunk in their teeth.

Chasing versus biting

The researchers then blocked the action of each set of neurons in turn. When they blocked the neurons responsible for prey pursuit, the mice were slow to chase but still able to bite. Conversely, when they blocked the biting neurons, the mice pursued the prey but could not deliver a killing bite.

The next step will be to determine what activates these two groups of neurons in real life, says de Araujo. “We don’t know yet,” he says. “But behavioural studies suggest that visual cues, especially small moving objects, are critical for triggering predatory sequences.”

The amygdala receives sensory information about visual cues, smell and sounds, so it makes sense for it to be involved in predatory hunting, says Elena Bagley at the University of Sydney, Australia. “It gets the right information to determine whether the prey is there, and then projects to regions that control motor function,” she says.

It’s likely that the amygdala evolved these functions when vertebrates first developed necks and jaws, which allowed more effective hunting and killing, says de Araujo. “The reconfiguration placed jawed predators at the top of the food chain.”